The present invention relates to an aerial drop cable with reduced weight and improved specific strength. In particular, the present invention relates to an optical fiber aerial drop cable including reinforcement strength members containing high modulus materials such as poly(p-phenylene-2,6-bezobisoxazole) or carbon fibers disposed on opposites sides of a central cavity containing one or more loose optical fibers wherein the strength members protect the optical fibers from compressive loads.
With the advent of new technologies and lower prices, the introduction of fiber optic installation in residential homes or xe2x80x9cFiber-To-The-Homexe2x80x9d (FTTH) is coming closer to reality. In a passive optical network, small optical cables containing only a few fibers will be deployed directly onto customer premises for providing video, data and voice connections with superior quality and bandwidth. The optical cables need to be designed with appropriate materials so that long term fiber and cable reliability are obtained at a cost that is acceptable for the distribution market.
Conventional copper cables have limited data transmission bandwidth and are subject to electromagnetic interference. Conventional optical fiber cables are designed for different applications and thus, do not have the features which are required for FTTH applications such as compatibility with existing hardware, self-support over large distances, and low flammability. In the past, plastic compounds which are typically used in copper drop cables have not been selected to ensure reliability for fiber optic telecommunication cables. For example, the carbon black content and U.V. absorption requirements for copper drop cables are far lower than the requirements for fiber optic cables. Polyvinyl chloride (PVC) jacketing compounds for copper drop cables contain only 0.5% carbon black by weight, whereas Bellcore GR-20 Issue 2 standards require 2.6%xc2x10.25% carbon black and an ultraviolet (U.V.) absorption coefficient of at least 400 at 375 nm. A high U.V. absorption coefficient is required to ensure U.V. resistance and a long service lifetime for the more expensive fiber optic cables and to protect light sensitive fibers from U.V. radiation when thin jackets are used. Additionally, materials that are non-reactive with optical fiber performance and/or fiber coating reliability must be selected. Use of traditional phthalate plasticized PVC materials can result in plasticizer migration to the fiber coatings which can result in a decrease in coating adhesion (as measured by coating strip force) and possible coating delamination resulting in catastrophic fiber failure.
Unlike copper drop cables, the presence of water and hydrogen in fiber optic drop cables is a significant concern. In particular, the presence of water or hydrogen in fiber optic drop cables can result in attenuation increases. Therefore, the fiber optic drop cables must be designed to prevent water ingression into the cable and the fiber optic drop cables must utilize materials that do not generate or release hydrogen.
Conventional aerial drop cables may be reinforced by metallic materials such as steel or copper, or non-conductive materials such as carbon fibers, aramid fibers, or glass reinforced epoxy rods. For example, U.S. Pat. No. 4,199,225 discloses an optical cable which utilizes a pair of steel or carbon fiber reinforcing wires disposed on opposite sides of a bore housing optical fibers in order to provide longitudinal support and protection against a crushing force applied to the optical cable. U.S. Pat. No. 4,199,225 discloses an optical cable which utilizes a pair of reinforcing members such as steel wire or carbon fiber disposed on opposite sides of a bore housing optical fibers in order to provide longitudinal support and protection against a crushing force applied to the optical cable. U.S. Pat. No. 5,673,352 discloses a fiber optic micro cable which includes reinforcing members manufactured of metal wire or non-conductive materials such as fiberglass, reinforced plastic or other dielectric materials. Similarly, U.S. Pat. No. 4,761,053 discloses an aerial service wire which utilizes a pair of strength members composed of a fibrous stranded material such as fiberglass or aramid fibers which are impregnated with a plastic material.
Aramid reinforced rods provide a high modulus and tenacity (e.g., Kevlar(copyright) aramid fiber manufactured by Du Pont Corporation has a modulus of 120 GPa and a tenacity of 24 g/D). Although aramid fibers are relatively expensive, aramid reinforced rods provide a higher modulus and lower weight than glass reinforced rods. Currently, aramid is the highest specific modulus material which is commercially available and utilized for reinforcement of composite strength members in dielectric cables.
The self-support span length of an aerial drop cable is very sensitive to the size and weight of the cable. As a result, attempts to achieve a longer span length by increasing the size of a reinforcement member (e.g., glass reinforced epoxy) may be compromised by increases in the cable weight and size. In particular, conventional cable materials may have a density greater than 1.2 g/cc which requires the use of heavier reinforcing members due to the higher cable weight. Moreover, the higher density increases the overall cable weight and cost and decreases cable flexibility. Accordingly, in order to maintain the cable elongation in a limited range, which is critical for fiber long term reliability, the only way to achieve long span requirements is by using higher modulus materials in the reinforcing members.
In view of the disadvantages of conventional aerial drop cables, it is an object of the present invention to provide an aerial drop cable using all dielectric components with reduced weight and size in order to increase span length and decrease ice and wind loading.
It is a further object of the present invention to provide an aerial drop cable with reduced bending strain for improved cable routability.
The present invention is adapted to achieve the foregoing objects. In accomplishing these objects, the present invention provides an aerial drop cable comprising a jacket surrounding a cavity containing a least one loosely housed optical fiber and a pair of reinforcing members composed of a high modulus material.
According to the present invention, the jacket comprises a central portion in which the cavity is disposed and two end portions in which the reinforcing members are disposed. In order to protect the optical fiber within the cavity from lateral compressive forces, the vertical thickness of the central portion of the cavity is smaller than the vertical thickness of the end portions and the cross-sectional area of the reinforcing members is larger than the cross-sectional area of the central cavity. As a result, when a lateral compressive force is applied to the aerial drop cable, the compressive force is absorbed by the end portions of the jacket and the reinforcing members. Further, by loosely housing the optical fiber in the cavity, the optical fiber is provided with freedom of movement and thus, is less prone to the bending losses introduced by stresses imposed on the cable in an outside environment.
In accordance with a preferred embodiment of the present invention, the reinforcing members are composed of polybenzoxazole (PBO) fibers. As a result, the diameter of the PBO reinforcing members is reduced as compared with conventional reinforcing members composed of aramid, metal or glass thereby providing a substantial reduction in the amount of jacketing material and the weight of the cable. Further, the bending strain of the cable utilizing PBO reinforcing members is substantially reduced for an equivalent bending radius as compared with cables utilizing conventional reinforcing members due to the smaller diameter of the reinforcing members- Therefore, the bending radius of the cable is reduced providing greater flexibility for routing the cable indoors.